Synthesis gas is composed primarily of hydrogen and carbon monoxide. Generally the H.sub.2 /CO ratio is from about 0.6 to 6. The production of synthesis gas from light hydrocarbons such as methane and natural gas by partial oxidation is known. The present invention describes a process for the preparation of synthesis gas from methane and natural gas by oxidation in an electrochemical reactor. In addition to the synthesis gas, electricity also may be generated as a by-product of the process of the invention.
Electrogenerative and voltameiotic processes are reviewed and compared with conventional electrochemical and heterogeneous processes in Ind. Eng. Chem. Process Dev., Vol. 18, No. 4, pp. 567-579. Oxidation reactions are discussed at p. 576.
Methane and natural gas have been subjected to chemical reactions conducted in electrocatalytic cells of various types, and various products have been isolated. Otsuka et al in Chemistry Letters, (Japan), pages 319-322, 1985, describe the conversion of methane to ethane/ethylene in an electrocatalytic cell using silver and silver/bismuth oxide as the anode materials and a temperature of 973.degree. F. (about 700.degree. C.). In an earlier publication, Bull. Chem. Soc. Jpn., 57, 3286-3289 (1984), the same authors described steam reforming of hydrocarbons through a wall of stabilized zirconia which acts as a hydrogen separator. The desired product is hydrogen with a minimum of carbon dioxide, carbon monoxide or hydrocarbons.
In Patent Early Disclosure No 1986-30,688 (Japan), Otsuka and Morikawa describe a method for manufacturing higher hydrocarbons from a lower hydrocarbon via oxidative dehydrogenation conducted in an electrolytic cell. The temperature of the oxygen supplied to the cell is from 300.degree.-1000.degree. C.
More recently, Seimanides and Stoukides reported on the oxidation of methane in a solid electrolyte cell using catalysts such as silver and lithium/magnesium oxide-silver. Ethylene, ethane, carbon monoxide and carbon dioxide were the main products. Electrochemical techniques were applied to increase the selectivity to C.sub.2 products (Preprint, AIChE Meeting, Miami, Fla., November, 1986).
Otsuka et al Chemistry Letter (Japan), 1985, 499-500 describe the selective oxidation of methane to ethane and/or ethylene in a conventional heterogeneous catalytic reactor. A low pressure mixture of oxygen and methane in helium is passed over a metal oxide catalyst at 700.degree. C. Among the metal oxides described as active for this reaction are included rare earth, main group metals and transition metals.
The electrocatalytic reactivity of hydrocarbons on a zirconia electrolyte surface is described by B. C. Nguyen, T. A. Lin and D. M. Mason in J. Electrochem Soc.: Electrochemical Science and Technology, September, 1986, pp. 1807-1815.
Numerous publications describe the complete oxidation of methane to carbon dioxide and water in fuel cells. These processes are not designed to be chemical processes, but rather to generate electricity from a fuel gas and air (or oxygen). The selectivity of these processes is designed for complete combustion rather than partial combustion.